Differential pressure operated switch

ABSTRACT

A pressure responsive control device including a housing, a control switch unit and switch operating actuator, and a controller unit is disclosed for controlling operation of a refrigeration defrosting device. The actuator includes a bladder formed of thin film material which changes volume when a differential fluid pressure exists across the bladder. The bladder and switch unit are such that bladder volume changes are accompanied by minimal losses thus enabling the controller unit to be enabled in response to extremely slight differential pressures.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to condition responsive control devices ormore particularly relates to control devices which are sensitive toextremely small levels of differential fluid pressure.

Control devices which respond to sensed differential fluid pressurechanges are of various kinds and types. Such control devices havefrequently included electrical switches, fluid flow controlling valves,or other such devices, operated by differential fluid pressure whichevidences the existence of some physical condition.

There are a number of environments in which it is desirable to be ableto sense extremely small differential fluid pressures and perform acontrol function. One example of such an environment is in a typicalhousehold refrigerator having a small fan for circulating air acrossair-chilling refrigeration coils. Moisture in the air tends to condenseon the chilling coils and forms ice, or frost, which impedes the abilityof the coils to chill the air and thus reduces the efficiency of therefrigerator. The efficiency of the refrigeration unit can be maximizedby promptly sensing a build-up of ice or frost on the coils and"defrosting" the refrigerator.

The continued formation of ice or frost on the coils gradually restrictsthe flow rate of air across them and therefore the presence of ice onthe coils is evidenced by a change in differential air pressure acrossthe coils. Accordingly, detecting the differential pressure of the airflowing across refrigeration coils is one way of determining when therefrigerator should be defrosted. Control devices have been proposed forperforming this function.

In environments of the character referred to the control device must beoperated by forces produced by small differential pressures. In order toeffectively respond to small differential pressures the control devicemust either be constructed so that the differential pressure isdistributed over a relatively large area to produce a correspondinglylarge control operating force, or be capable of responding accuratelyand reliably to extremely small forces.

Generally speaking, control devices used in mass produced equipment,such as refrigerators, cannot be large because of space limitations inthe equipment. Hence, control devices which rely on large pressure areasupon which small differential pressures act to exert actuating forcesare not practical.

Reasonably small-sized control devices for sensing differentialpressures have frequently utilized a flexible diaphragm across whichdifferential pressure is established to impart operating force to thediaphragm. The diaphragms are usually constructed from a rubber orrubber-like plastic material and are secured in place along their outerperipheries. A plate or pad is generally secured to the diaphragm fortransmitting actuating forces from the diaphragm to a control member,such as a valve or switch element. Prior art control devices employingdiaphragm-type actuators are disclosed, for example, by U.S. Pat. Nos.2,229,740 (issued to Helmore on Jan. 28, 1941); 3,066,496 (issued toJokela on Dec. 4, 1962); 3,300,703 (issued to Gold, et al., on Jan. 24,1967); and 3,359,388 (issued to Houser, et al., on Dec. 19, 1967).

Control devices employing inflatable actuators have also been proposed.These devices, in essence, have employed balloon-like actuator elementsinstead of diaphragms and the actuator elements have been resilientlyexpanded, by applied differential pressure, into engagement with movablecontrol members. The control members in turn have performed controlfunctions by sliding or pivoting movement imparted to them by theactuators. Prior art devices of this general type are disclosed, forexample, in U.S. Pat. Nos. 1,942,040 (issued to Wolff on Jan. 22, 1934);2,795,668 (issued to Pucket on June 11, 1957); and 3,247,341 (issued toKizilos on Apr. 19, 1966).

Operation of these kinds of control devices by extremely smalldifferential pressures (for example 0.05 inches of water or less whichcorresponds to 0.002 psi or less) has not been possible because of anumber of problems. Firstly, the the mass of the diaphragm, plate and/orthe associated components which must be moved during operation of thecontrol device has been so great relative to the available force thatsuch controls have lacked adequate sensitivity. Secondly, the existenceof frictional losses in the devices has been relatively great (comparedto the available forces). Thirdly, frictional losses have not beenconsistent from cycle to cycle thus reducing the accuracy andrepeatability of the devices. Furthermore, many devices are positionsensitive in that the differential pressure levels to which they respondchange depending upon the positional orientation of the device(primarily because of the varying effects of gravity on the pressureresponsive element and associated components). Still further, many priorart control devices have been constructed so that increasingly greaterdifferential pressures are required to be sensed in order to supplyenough force to complete a control function of the device.

The kinds of problems referred to have prevented the introduction ofsmall, highly sensitive, relatively inexpensive control devices forgeneral purpose use in mass produced appliances.

SUMMARY OF THE INVENTION

The present invention provides a new and improved differential fluidpressure responsive control device which is constructed and arranged tosense and respond reliably to extremely small differential pressures yetis compact, composed of relatively inexpensive readily availablecomponents and inexpensively manufactured.

According to a preferred embodiment of the invention, the new controldevice includes a support housing having a reaction face, a controlmember supported for movement relative to the reaction face and adifferential pressure responsive actuator reacting between the face andthe control member to move the control member.

The actuator includes a bladder formed of supple exceedingly thin filmmaterial and a conduit member for communicating the interior of thebladder with a source of fluid pressure which may vary relative to thefluid pressure ambient the bladder. The bladder volume changes inresponse to differential pressures between the source and ambient fluidsand as the bladder volume expands the control member is engaged by thebladder. The differential pressure acts on the control member via thebladder wall resulting in the control member being moved to initiate acontrol function.

The bladder is preferably a flattenable bag-like structure having onemajor face extending along the reaction surface, an outer peripherywhich, at least for the most part, is unsupported by and freely movablewith respect to the support housing, and a second major face which isengageable with the control member upon expansion of the bladder. Thebladder volume is relatively large so that the bladder is expansible tomove the control member to its limit of motion without any appreciablestrentching of the film-like bladder material.

The bladder construction minimizes losses during inflation thusmaximizing the accuracy and sensitivity of the control device to smalldifferential pressures. The bladder material is a thin (e.g., 1 to 1.25mils), low density, biaxially oriented film which is extremely supple.The pliability of the material is substantially temperature independentthroughout the normal atmospheric temperature range (e.g., from -20 to100° F). The material is static free in that it will not cling to itselfas a result of the presence of static electric charge, and has a lowcoefficient of sliding friction of the control member. Thesecharacteristics assure that losses such as those due to friction inflexing the bladder, frictional resistance to relative motion betweenthe bladder and the control member, and losses accompanying work done inexpanding the bladder are very slight and substantially the same fromcycle to cycle of the device.

The actuator conduit member secures the bladder in position to thehousing without requiring substantial restraint of the bladder peripheryrelative to the housing. The conduit member and bladder are constructedfor quick and easy assembly and are related to the housing so thatleakage to or from the bladder is effectively prevented when theactuator is assembled to the housing.

The bladder and control member cooperate so that as the bladder expands,a force amplifying effect is achieved by virtue of the bladdercontacting the control member over pressure areas which increase as thepressure differential increases. The control member is preferably formedby sheet material which presents a substantial area for engagement bythe actuator. As the bladder inflates, one of its faces initiallytouches the control member along a small area. A small actuatingpressure force is transmitted to the control member causing a slightmovement of the control member. Further expansion of the bladder volume,resulting from an increase in sensed differential pressure, causes thebladder to engage the control member over an increasingly great area.This relationship results in increasingly greater actuating forces beingapplied to the control member over increasingly greater area even thoughthe control member tends to move away from the bladder.

The control member is preferably formed of spring material and iscantilevered to the housing. The application of actuating force to thecontrol member from the fluid pressure in the bladder is distributedover a substantial length of the control member. The applied force thusarcuately flexes the control member along an increasing portion of itsextent as the bladder inflates and effectively amplifies the motion ofthe projecting control member end. The cantilevered support of thecontrol member eliminates frictional losses which might otherwise bepresent if the control member were hinged to the housing.

The control member is formed with cutout portions so that the weight ofthe control member is minimized. The bladder is permitted to expandthrough the cutouts beyond the plane of the control member so that thetensile strength of the bladder material participates to some extent inmoving the control member.

In a preferred embodiment of the invention the control device includesan electrical control switch unit and the control member forms a movablecontact arm of the switch. The control member is secured at one end tothe housing and projects along the bladder. As the bladder volumechanges, the projecting end of the control member, which forms a switchcontact, moves relative to the housing to open and close the switch.

The control member is associated with a second projecting resilientswitch member which is constructed and arranged so that regardless ofthe mounting position of the control device, deflections of the switchmembers caused by gravity forces are consistent. Hence, the differentialpressure level at which the control device is set to operate is notsubstantially affected by the orientation of the device.

The switch member is adjustably supported relative to the housing and apositioning mechanism for the switch member is provided which permitsprecise position adjustment of the second switch member relative to thecontrol member. This in turn enables precise setting of the differentialpressure levels to which the device responds.

Other features and advantages of the invention will become apparent fromthe following detailed description of a preferred embodiment made withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a portion of a refrigration systemembodying the present invention;

FIG. 2 is an elevational view of a control device embodying the presentinvention having portions broken away and portions illustrated incross-section;

FIG. 3 is a cross-sectional view of the device illustrated by FIG. 2seen approximately from the plane indicated by the line 3--3 of FIG. 2;

FIG. 4 is a cross-sectional view of the device of FIG. 2 seenapproximately from planes indicated by the line 4--4 of FIG. 2.

FIG. 5 is a schematic illustration of a portion of a device shown inFIG. 2;

FIG. 6 is a cross-sectional view seen approximately from the planeindicated by the line 6--6 of FIG. 5; and,

FIG. 7 is an enlarged cross-sectional view of a portion of the device ofFIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT

A portion of a refrigeration system 10 embodying the invention isillustrated by FIG. 1 of the drawings and includes a refrigeration coil12 stationed in a duct, or passage, 14 through which air is circulatedby a fan 16, a defroster 18 for moving ice or frost from the coil 12 anda control device 20 for initiating operation of the defroster.

The refrigeration unit 10 can be of any suitable construction but forthe purpose of this description it will be assumed that therefrigeration system includes a compressor/condenser/evaporator-typemechanical refrigeration unit and that the refrigeration coil 12 isformed by the evaporator of the unit. Typical refrigeration coils areformed by a serpentine heating-conducting metal tube through whichrefrigerant flows. The tube is maintained well below the freezingtemperature of water by the circulating refrigerant. As air circulatesacross the coil the air is chilled and moisture in the air condenses onthe cold surface of the coil. The condensed moisture freezes and after atime ice or frost accumulates on the coils and reduces the efficiency ofthe refrigeration unit.

As the ice or frost builds up on the coil 12, the flow of air across thecoil is impeded. Impeding the flow of air across the coil 12 causes theair pressure in the duct upstream from the coil to increase above theduct air pressure downstream from the coil. The extent of thedifferential pressure across the coil is a measure of the amount of iceor frost present on the coil.

The control device 20 is a differential pressure responsive controldevice which is in pressure communication with air upstream anddownstream from the coil 12 via pressure transmitting lines 22, 24. Whensufficient ice or frost accumulates on the heat exchanger 12 theresultant differential pressure detected by the device 20 causes thedevice 20 to initiate operation of the defroster 18. When defrosting isconcluded another suitable device, such as a thermostat, terminatesoperation of the defroster.

The defroster 18 may be of any suitable or conventional construction,such as a refrigerant flow reversing valve and associated components,and is not illustrated or described in detail here. Suffice it to saythat the defroster is constructed and arranged so that its operation isinitiated electrically.

One preferred construction of the control device 20 is illustrated byFIGS. 2-4 of the drawings and comprises a housing assembly 30 supportinga differential pressure responsive actuator 34 and associated controlswitch unit 35, and a defroster controller unit 36. The actuator 34actuates the control switch 35 in response to the existance of apredetermined low-level differential pressure across the refrigerationcoil 12. The control switch 35 is thus actuated when a predeterminedamount of frost or ice is sensed on the coil 12 an in turn operates thedefroster controller 36. The defroster controller responds to operationof the switch unit 35 by initiating operation of the defroster 18.

The housing assembly 30 supports the actuator 34 and control switch unit35 in one housing chamber and the controller 36 in a second housingchamber, the housing chambers being sealed from each other andelectrically isolated. The assembly 30 preferably includes a housingbody 42 molded from a dimensionally stable, high dielectric plasticmaterial (e.g., Bakelite) and which defines oppositely facing recesses44, 46. The recess 44 is defined by a reaction surface 50 and aperipherally extending wall 52 disposed about the surface 50. Moldedpressure tops 56, 57 project from the housing body 42 and definerespective passages 58, 59 by which air pressure is communicated to therecess 44 via the lines 22, 24, respectively. In the illustratedembodiment of the invention a conventional air filter element 60 isdisposed in the passage 59 to prevent particulate matter from enteringthe recess 44.

The actuator 34 and the switch unit 35 are assembled to the housing bodyin the recess 44 and a plastic cover plate 54 is sealed in place overthe open side of the recess remote from the surface 50.

The recess 46 is formed by a base wall portion 62 and a peripheral wall64 which extends about the base wall 62. The controller 36 is assembledin the recess 46 and the recess is closed by a dielectric plastic coverplate 66.

The illustrated housing body 42 is provided with mounting slots 68 whichreceive headed mounting screws or the like (not shown) for clamping thecontrol device in a desired mounting location.

The actuator 34 operates the switch unit 35 in response to extremelylow-level, sensed differential pressures and in the preferred embodimentof the invention is formed by an inflatable bladder 70 and an associatedconduit member 72 connecting the bladder 70 to the housing assembly 30.The bladder 70 is a flattenable bag-like element formed by a pair ofrectangular bladder wall forming sheets 74, 76 of film material whichare heat sealed together along a rectagular peripheral edge 78. The edge78 is disposed within the housing assembly 30 and unrestrained againstmotion relative to the housing assembly.

The conduit member 72 anchors the bladder 70 to the housing assembly 30without effectively constraining the bladder for any particular motionrelative to the housing and also communicates the interior of thebladder to the air pressure on the upstream side of the refrigerationcoil. In the preferred embodiment of the invention the conduit member 72includes a tubular cylindrical body 80 and an enlarged flange-like head82 (See FIG. 7). The head 82 is disposed within the bladder 70 and thebody 80 projects through a circular aperture 84 in the bladder wall 76.The aperture 84 has a substantially smaller diameter than the head 82and the conduit member 72 and bladder are assembled together byinserting the head 82 through the aperture 84 much like passing a buttonthrough a buttonhole.

When the conduit member 72 is assembled to the bladder the aperture 84may resiliently grip the periphery of the body 80 to form a seal betweenthe bladder and the conduit member. The body projects into andfrictionally grips the passageway 58 in the tap 56 and the material ofthe sheet 76 is interposed and gripped between the head 82 and a lip 106formed on the reaction surface 50 peripherally about the passage 58.

Since the housing chamber ambient the bladder 70 is communicated to theduct 14 downstream from the refrigeration coil 12 and the interior ofthe bladder 70 is communicated to the duct 14 upstream from the coil,resistance to air flow across the refrigeration coil causes the pressurewithin the bladder to exceed the pressure ambient the bladder and thebladder inflates.

The bladder is interposed between the reaction surface 50 and the switchunit 35 and as it inflates it reacts between the surface 50 and theswitch unit 35 to actuate the switch unit. The switch unit 35 includes acontrol member 120 actuable by the actuator 34, a contact member 122which is engaged by the member 120 for rendering the switch unitconductive and an adjusting mechanism 123 for the contact member 122.The control member 120 is engaged by the inflating bladder and is movedtoward the contact member 122 until, at a predetermined senseddifferential pressure level, the members 120, 122 are in contact thus"closing" the switch unit 35. The adjusting mechanism 123 positions thecontact member 122 to govern the differential pressure level at whichthe members 120, 122 engage.

The control member 120 is preferably formed by a thin sheet of beryliumcopper spring material having cutout portions which define parallelmounting legs 124, 125 which are respectively attached to the housingbody 122 by fasteners 128. A circularly curved end portion 130 extendsbetween the projecting ends of the legs 124, 125 and a lateral strut 132connects the mounting legs at a location between their ends. A switchcontact element 134 is secured to the end portion 130.

The control member 120 has an undeflected position in which it projectsfrom its connection to the housing assembly 30 generally parallel to thereaction surface 50. As the member 120 is engaged by the bladder 70 tolegs 124, 125 are resiliently deflected by the applied differentialpressure force so that the contact element 134 is moved toward thecontact member 122. The degree of deflection of the legs 124, 125 is adirect function of the level of differential pressure sensed by thebladder 70. The member 120 is self-biased to its undeflected position sothat it resists movement by the bladder 70.

The construction of the control member 120 and the bladder 70 and theircoaction contributes substantially to the ability of the device 20 tosense and respond to differential pressures as small as 0.02 inches ofwater (about 0.00072 psi). In particular, the actuator 34 and thecontrol member 120 are constructed to maximize the efficiency with whichthe switching unit 35 is operated by the actuator 34. The preferredbladder material is a biaxially oriented ethylene vinyl acetatecopolymer which is extremely limp or pliant at least at ambienttemperatures ranging from -20° F to 100° F. The suppleness of thebladder material is such that the bladder has virtually no structuralstrength except in tension (the material is in tension, for example,when the bladder is inflated against some resistance). When the bladderis inflated the peripheral edge 78 buckless as the bladder walls 74, 76move away from each other and buckling also occurs in the bladder walls74, 76 along the edge 78. Because of its limpness, pliability and slightwall thickness (1-1.25 mils) the bladder buckles and bends whileexpanding without incurring any significant energy losses attributableto deformation of the bladder.

The bladder material is also "static free" in that it will not cling toitself as a result of static electrical charges on the bladder faces.This is important because the faces of the bladder are easily separatedduring inflation without requiring work or energy losses attendantovercoming attractive static forces acting on the bladder.

The bladder material likewise has a low coefficient of sliding frictionwhen engaged with the control member 120 and the reaction surface 50.This permits the materials to slide readily relative to one anotherduring inflation of the bladder without significant losses.

The cantilevered connection of the member 120 to the housing body 42permits resilient deflecting movement of the member relative to thehousing with minimal frictional losses associated with displacement ofthe member.

The bladder 70 and control member 120 are constructed to minimize theirweights thus enabling greater sensitivity to small actuating forces. Therectangular bladder 70 is preferably about 2 inches long by 11/2 incheswide with the wall thickness of the bladder material being, as notedabove, on the order of 0.001 inches. Hence the inflation of the bladderand the attendant shifting of the center of gravity of the bladder doesnot require significant force or work. The sheet material forming thecontrol member 120 is formed to provide large area openings between themounting legs which reduces the weight of the member. The thickness ofthe preferred control member is about 0.008 inches which minimizes themember weight yet assures adequate structural strength.

The relationship of the bladder 70 and the control member 120 alsoprovides for a pressure force amplification effect which is achievedduring actuation of the switch unit 35.

The bladder volume is such that if the control member 120 were notpresent in the housing assembly the bladder 70 could expand through thespace occupied by the member 120 before the applied differentialpressure created tension in the bladder walls. Therefore expansion ofthe bladder 70 to a volume which is sufficient to close the switch unit35 can occur without the bladder walls being stressed. The bladder maythus be thought of as more or less "slack" during actuation of thecontrol member 120 because the only tension in the bladder walls is thatdue to resilient engagement with the member 120.

Referring to FIGS. 2, 5 and 6, the control member mounting legs 124, 125and the strut 132 define substantial areas confronting the bladder 70 sothat when the bladder engages the control member 120 the pressure withinthe bladder is exerted over the area of the control member which isengaged by the bladder. When a differential pressure is initially sensedthe bladder 70 is expanded to the position generally shown in FIG. 3.The bladder is extremely lightly engaged between the member 120 and thereaction face 50. The member 120 resiliently deflects very slightlybecause of its engagement by the bladder 70 and the position of themember 120 stabilizes so long as no further differential pressureincreases are sensed.

Assuming that the sensed differential pressure increases, the bladder 70is inflated further resulting in the bladder 70 engaging the legs 124,125 and the strut 132 over a substantially increased contact area (seeFIG. 5). The increasing area of engagement occurs because of the "slack"nature of the bladder and its general configuration and in spite of themember 120 being deflected further away from its initial position.Because the member 120 is engaged over an increasingly large area ofcontact by the bladder and because the pressure differential across thebladder likewise increases as the area of contact increases the coactionbetween the bladder and the control member 120 effectively provides fora multiplication of the available actuating force.

In addition to the force multiplication effect, the relative slacknessof the bladder material permits the bladder to "billow," or bulge,through the open areas of the member 120 and exert supplemental albeitsmall actuating forces on the control member. The bladder wall 74 bulgesthrough the plane of the member 120 (see FIGS. 5 and 6) with the shapesof the bulges being generally arcuate. To the extent that the bladdermaterial is tensioned, the tensile forces (indicated by arrows, FIGS. 5and 6) in the bladder material are directed tangent to the bladdercurvature and thus tend to actuate the member 120 toward engagement withthe member 122. These forces are small compared to the differentialpressure forces exerted on the member 120 but they neverthelesscontribute towards actuation of the switch unit 35.

Actuation of the member 120 by the bladder 70 is further accompanied byan amplification effect in the displacement of the projecting end of themember 120. Since the actuating force applied to the member 120 by thebladder 70 is distributed over a substantial length of the member 120the member 120 is deflected all along the area of application of theactuating force. Hence the only location along the member 120 which isnot resiliently deflected is that portion of the member 120 whichprojects beyond contact with the bladder 70. The progressivelyincreasing amounts of deflection of the control member 120 tend tomaximize the travel of the contact element 134 for a given senseddifferential pressure level.

When a predetermined differential pressure level has been applied acrossthe bladder 70 the control member 120 flexes from its initial positionto engage the contact member 122 so that the switch unit 35 conducts. Inthe preferred embodiment of the invention the contact member 122includes a base section 140 anchored to the housing body 42 by asuitable fastener, a projecting cantilever arm section 142, and abridging section 143 extending between the base and arm sections. Thearm section 142 supports a contact element 144 at its projecting end andwhen the member 120 is deflected sufficiently the contact element 134 atthe end of the member 120 engages the slides on the contact element 144.

The switch unit 35 is constructed and arranged so that the controldevice 20 can be installed in various orientations without substantiallyeffecting the differential pressure level at which the switch unit 35 isactuated. As is best illustrated by FIG. 2 of the drawings thecantilever arm section 142 includes an enlarged tip portion at itsprojecting end and a relatively narrow deflectable spring portionbetween the bridging section 143 and the tip portion. The shapes andsizes of the tip and spring portions are such that the deflection of thearm section 142 under the influence of gravity forces is essentially thesame as the deflection of the control member 120 by the same forces.Hence no matter which way the device 20 is orientated with respect togravity, the gravity forces applied to the members 120 and 122 causethem to deflect substantially equally. The contact elements 134, 144thus remain spaced apart a constant distance and the setting of thecontrol is virtually unaffected.

When changing the differential pressure level to which the device 20responds is desirable, the contact member adjusting mechanism 123 isoperated to change the initial spacing between the contacts. In apreferred and illustrated embodiment of the invention, the mechanism 123is constructed and arranged to enable relatively fine positioning of thecontact member 122 quickly and easily. As shown by FIG. 3, the contactmember base section 140 and the arm section 142 extend generallyparallel to each other and the bridging section 143 extends between themmaking a small obtuse included angle with each. The housing body 42 hasa tapped opening 160 extending through it into the recess 44 with theaxis of the opening extending perpendicularly to the plane of the basesection 140. The bridging section 143 extends partly across the openingmaking a small acute included angle with the axis. An adjusting screw166 is threaded into the opening for movement along the axis and theprojecting end of the screw engages a plastic sheath 170 (FIG. 3) whichextends along the bridge section 143. When the screw is advanced intothe opening the bridging section is resiliently deflected toward aposition in which it is normal to the base section 140 and the contactelement 144 is moved away from the contact element 134 to increase thedifferential pressure level to which the control device responds. Whenthe screw is retracted the bridging section 143 is resiliently inclinedfurther across the axis and the contact elements are moved closertogether so that the device 20 responds to a lower level differentialpressure.

Relatively fine manual adjustment of the device setting is accomplishedquickly and easily. The driving end of the screw is accessible forturning by an ordinary screw driver externally of the housing assembly30. The screw and the plastic sheath have basically a worm and gearrelationship with a substantial amount of turning motion of the screwaccompanied by a relatively small amount of relative motion of thecontact elements. The screw threads are preferably relatively fine sothat precise adjustment of the device setting can be easily achieved.

The construction of the control switch unit 35 is such that ambienttemperature changes do not materially affect its "setting." Referring toFIGS. 3 and 4 it can be seen that the housing body portion to which thecontrol member 120 is attached is considerably thicker than the housingbody portion to which the switch arm 120 is attached (measured, forexample, from the plane of the reaction surface 50). When ambienttemperature changes alter the temperature the plastic housing bodymaterial, the material expands or contracts (depending on the directionand extent of the ambient temperature change) and the mounting locationof the member 120 is shifted a greater distance relative to the plane ofthe surface 50 than is the mounting location of the member 122.

This differential shifting of the mounting locations tends to cause thecontact element 134 to be displaced toward or away from the location ofthe contact element 144. If the ambient temperature increases theelement 134 tends to shift toward the element 144 and if the temperaturedecreases the element 134 tends to shift away from the element 144. Theextent of contact shifting caused by normally expectable ambienttemperature variations can, if not compensated, cause an appreciable,and undesirable, change in the differential pressure level responded toby the device 20.

The mechanism 123 functions to compensate the switch unit 35 for suchambient temperature changes. When the adjusting screw temperaturechanges, the perimeter of the screw 166 shifts in a direction generallytransverse to the direction of extent of the bridging section 143. Thisresults in the contact element 144 being shifted toward or away from thecontact element 134. The extent of the transverse shifting of the screwperimeter, while small compared to the extent of the relative shiftingbetween the members 120, 122, is amplified by the deflection of themember 122. As a result, the displacement of the contact element 134caused by a ambient temperature changes is closely matched, in directionand extent, by the temperature responsive displacement of the element144.

In the preferred embodiment of the invention, motion of the projectingend of the switch member 122 is damped to prevent the member 122 fromvibrating substantially. The mass and spring systems provided by the tipand arm sections of the member 122 can vibrate substantially if thedevice 20 is connected to equipment which imparts vibrational forces tothe housing 30. Vibrations of the contact member 122 can result inundesirable chattering of the contact elements 134, 144 as the switchunit 35 approaches its closed condition. A damper element 180 (see FIG.3) formed by a piece of yieldable foam plastic material is disposedbetween the housing assembly 30 and the projecting tip of the contactmember 122 to dampen the vibrations. The illustrated damper 180 is fixedto the cover plate 54 by a suitable adhesive and lacks the structuralstrength to interfere materially with position changes of the member122.

The control member 120 and the contact member 122 are electricallyconnected between the controller unit 36 and a grounded terminal 182 ofthe control device so that when the members 120, 122 are engaged anenergizing circuit for the controller 36 can be completed through themembers 120, 122.

The controller unit 36 may be of any suitable or conventionalconstruction. In the embodiment of the invention illustrated by FIG. 2the controller unit 36 is formed by a snap acting microswitch 200 and aswitch operator 202 both of which are disposed in the housing assembly30. The microswitch 200 initiates operation of the defroster 18 whenactuated by the operator 202. The operator 202 in in turn controlled bythe switch unit 35.

The microswitch includes the usual operating button 204 which controlsthe position of the snap switch contacts. The microswitch contacts areconnected between terminals 206, 208 which are carried by the housingassembly 30 and form part of an energizing circuit for the defroster 18.When the button 204 is depressed by the operator 202 the microswitchcontacts close to initiate operation of the defroster 18.

The operator 202 can be of any suitable or conventional construction andfor the purposes of this description is a heat motor which actuates themicroswitch under control of the switch unit 35. The heat motor isformed by an operating arm 220 supported by a pivot assembly 222 whichis driven by a bimetal blade 224 and associated blade heater 226.

The blade 224 is fixed to the pivot assembly at one end and engages thehousing assembly at is projecting end so that when the blade 224 warpsdue to differential expansion or contraction of its component metals thepivot assembly and the operating arm are moved. The blade heater 226 ispreferably formed by heater wires wound on the bimetal blade and theillustrated heater is energized from alternate power supplies viaterminals 230, 232 through a circuit connected to ground through theswitch unit 35 and the ground terminal 182.

Whenever the switch unit 35 is in its conductive condition a heaterenergizing circuit is established through the control switch unit 35 sothat the bimetal blade is heated in accordance with the electrical powerpassing through the heater and the length of time the control switch 35is closed. When the bimetal blade is heated sufficiently, it moves thepivot assembly 222 and the operating arm 220 to depress the microswitchoperator button to initiate operation of the defroster 18.

While a single embodiment of a preferred form of the invention isillustrated and described herein in considerable detail the presentinvention is not to be considered limited to the precise constructionshown. Various adaptations, modifications and uses of the invention mayoccur to those skilled in the art and the intention is to cover all suchadaptations, modifications and uses which fall within the scope orspirit of the appended claims.

What is claimed is:
 1. A differential pressure responsive control devicecomprising:(a) a support housing; (b) a control switch member supportedby said support housing and movable relative to said housing betweenfirst and second positions, said member defined by a thin sheet ofelectrically conductive material cantilevered to said housing andresiliently deflectable between said positions, said member biasedtoward said first position; (c) a bladder operatively interposed betweensaid member and said housing to effect movement of said member to saidsecond position from said first position, said bladder formed at leastin part by supple film material and constructed and arranged to beinflatable up to a predetermined volume without resiliently deformingthe film material; (d) conduit means for communicating the interior ofsaid bladder to a source of fluid so that the bladder is inflated towardsaid predetermined volume when the fluid pressure level in said bladderexceeds the pressure ambient said bladder; (e) said member defining amajor face confronting said bladder and said supple film material ofsaid bladder engaging and conforming to said member face so thatdifferential pressure force is applied to said member for moving saidmember to said second position when the volume of said bladder issubstantially less than said predetermined volume.
 2. The device claimedin claim 1 wherein said bladder engages said member over a contact areawhich increases in size as said bladder inflates.
 3. The device claimedin claim 1 wherein said conduit means comprises a tubular body and saidbladder includes a wall defining an aperture through which said tubularbody extends, said conduit means further comprising an elementengageable with said bladder about said aperture for clamping saidbladder against said housing assembly.
 4. The device claimed in claim 1wherein said control switch member forms a moving contact of a switchunit, said switch unit including a switch arm mounted cantilever fashionto said support housing, said switch arm defining a spring section andan enlarged tip section.
 5. The device claimed in claim 4 furtherincluding adjusting means for controllably varying the position of saidswitch arm relative to said control switch member to adjust thedifferential pressure level at which said switch arm is engaged by saidmember.
 6. A control device responsive to differential fluid pressurecomprising:(a) a support housing assembly defining a chamber portion;(b) control switch means in said chamber portion comprising:(i) a firstswitch element cantilevered to said housing and movable from a firstposition to a second position relative to said housing in response tosensed differential pressure, said first element biased toward saidfirst position; (ii) a second switch element disposed in said housingfor engagement by said first switch element in one of said positions;and, (c) an actuator for said switch means comprising a bladder formedat least in part by a supple film of material disposed in said chamberfor effecting movement of said first switch element from said firstposition, said bladder changing volume in response to changing fluidpressure differentials between its interior and exterior and defining abladder wall portion of said supple film material which is engageablewith and conforms to said first switch element over a contact area whichincreases in size in direct relation to increases in the differentialfluid pressure.
 7. The control device claimed in claim 6 wherein saidbladder is formed by confronting sheets of said film material sealedtogether about their peripheries to define a peripheral edge disposed insaid chamber portion and freely movable relative to said housingassembly.
 8. The control device claimed in claim 7 wherein said bladdersheets are composed of ethylene vinyl acetate plastic having a thicknessof about 0.00125 inches.
 9. The control device claimed in claim 6wherein said first switch element is formed by a thin sheet of springmaterial defining at least a sheet-like mounting leg extending alongsaid bladder for engagement by said bladder wall portion.
 10. Adifferential pressure responsive control device comprising:(a) a supporthousing defining a reaction surface portion; (b) a control membercomprised at least in part of a sheet-like material supported by saidsupport housing and resiliently deflectable relative to said reactionsurface portion between first and second positions, said control memberself-biased toward said first position and providing a major surfacearea confronting said reaction surface; (c) a bladder interposed betweensaid control member and said reaction surface portion to effect movementof said control member to said second position, said bladder defining aflattenable bag-like structure having a periphery disposed between saidreaction surface and said control member which is unrestrained againstmovement relative to said housing; and (d) conduit means for exposingsaid bladder to differential fluid pressure for inflating said bladder;(e) said bladder defining a first wall defined at least in part by asupple film material for directly engaging and conforming to said majorsurface of said control member and transmitting pressure forces of thefluid in said bladder to said major surface via said film material and asecond, opposed wall for directly engaging said reaction surface, saidfirst wall engaging said major surface over a contact area whichincreases as the control member is deflected from said first to saidsecond position.